It seems like this is going to be a daily thing at this rate! Pinkie Pie has been added to 15.ai, following Rainbow Dash, Applejack, and Celestia.
As with the others, you can find her in the dropdown menus.
This update also includes improvements for the rest of the ponies, so feel free to try them again too!
Thanks to Reach, Riocin, and everyone else for the heads up.
